In a digital signal processor, an analog signal is converted to a digital signal by an AD (Analog to Digital) converter, and digital signal processing is performed on the digital signal, whereby, for example, a received waveform distorted in a transmission line can be compensated in a digital region. The digital signal processor is used in various technical fields, such as image processing, sound processing, wireless communication, and optical communication.
When performing the digital signal processing, if transient changes, such as pulse noise, occur in the digital signal during the digital signal processing, errors in the digital signal processing increase and the quality of an output signal is deteriorated. However, the influence of the transient change components on the digital signal processing can be reduced by, for example, averaging processing of the digital signal using a low pass filter.
For example, in a digital coherent receiver in an optical communication field, in order to reduce the influence of the transient changes, there is a method which, when performing equalization processing on a received waveform using a FIR (Finite Impulse Response) filter, increases the number of taps of FIR filters. Alternatively, there is a method which estimates the phase shift from the phase point of the phase modulation signal using an M-power algorithm, and when removing the phase shift, increases the cumulative number of signals (the number of taps) of the M-power algorithm.
Here, a digital coherent receiver which uses a coherent optical communication technique and a digital signal processing technique in combination will be described.
FIG. 5 shows a configuration example of a digital coherent transmission/reception system (Non-Patent Document 1).
In FIG. 5, the digital coherent transmission/reception system has a transmitter 100 which transmits a phase-modulated optical signal, and a digital coherent receiver 200 which receives and demodulates the optical signal transmitted through a transmission line. The digital coherent receiver 200 has a coherent receiver 210, an AD converter 220, and a digital signal processor 230. The coherent receiver 210 inputs the optical signal received from the transmission line and local light from an optical local oscillator 301 and converts the optical signal to an electrical signal by a coherent detection technique with high sensitivity. The AD converter 220 converts the electrical signal output from the coherent receiver 210 to a digital signal. The digital signal processor 230 performs digital signal processing on the digital signal output from the AD converter 220 and demodulates the digital signal while compensating for a received waveform distorted in the transmission line.
The digital signal processor 230 has an equalizer 231, a phase shift compensator 232, and a demodulator 233. The equalizer 231 equalizes waveform distortion of the input digital signal, and the phase shift of the waveform-equalized digital signal is compensated by the phase shift compensator 232. The demodulator 233 outputs the phase shift-compensated digital signal output from the phase shift compensator 232 as a symbol string. In this way, since the correction of the waveform distortion can be performed with a simple configuration, a large-capacity and high-speed transmission system can be realized.
The phase shift compensator 232 can estimate and correct the phase shift using, for example, the M-power algorithm (Non-Patent Document 2). Since the estimation range of the phase shift in the M-power algorithm is limited within the range of ±π/4 from a reference point for a QPSK (Quadrature Phase Shift Keying) signal, a phase shift outside the range cannot be estimated. A phenomenon in which the time continuity of the phase shift estimation values is not maintained is called a “cycle slip”, and signal quality is deteriorated. For example, when transient changes like pulse noise occur in the digital signal, time continuity is not maintained due to the error expansion of the digital signal processing, and as shown in FIG. 6, a cycle slip occurs.
As a countermeasure against the cycle slip, a method which performs logical differential coding on a transmission signal to prevent the propagation of the influence, or the like is used (Non-Patent Document 3). However, bit errors at the moment when the cycle slip occurs cannot be prevented. When one bit error occurs in differentially coded data, since the bit error is subjected to differential decoding as continuous two bit errors, transmission quality is deteriorated.    Non-Patent Document 1: S. J. Savory, “Digital filters for coherent optical receivers” Optics Express, vol. 16, no. 2, pp. 804-814, 2008    Non-Patent Document 2: S. Tsukamoto, Y. Ishikawa, and K. Kikuchi, “Optical Homodyne Receiver Comprising Phase and Polarization Diversities with Digital Signal Processing” Proc. ECOC, 2006    Non-Patent Document 3: T. Mizuochi, Y. Miyata, K. Kubo, T. Sugihara, K. Onohara and H. Yoshida, “Progress in Soft-Decision FEC” OSA/OFC/NFOEC, NWC2, 2011